FIG. 20A is a schematic top view of conventional acoustic wave sensor 101. FIG. 20B is a schematic cross-sectional view of acoustic wave sensor 101 at line 20B-20B shown in FIG. 20A. Acoustic wave sensor 101 includes piezoelectric substrate 102, transmitting electrode 103 provided on piezoelectric substrate 102 for exciting a main acoustic wave, receiving electrode 104 for receiving the main acoustic wave, insulating film 105 provided on piezoelectric substrate 102 for covering transmitting electrode 103 and receiving electrode 104, and reaction section 106 provided on a propagation region on insulating film 105 and between transmitting electrode 103 and receiving electrode 104 on insulating film 105.
When a substance (such as breath or sample liquid) which may possibly contain an object to be detected is injected onto reaction section 106, the acoustic wave sensor detects a change in a propagation characteristic of an acoustic wave due to the attaching of the object, thereby detecting the presence of the object or a concentration thereof.
A conventional acoustic wave sensor similar to acoustic wave sensor 101 is described in Patent Literature 1.
FIG. 21 is a schematic cross-sectional view of another conventional acoustic wave sensor 901. Acoustic wave sensor 901 includes piezoelectric substrate 902, transmitting electrode 903 and receiving electrode 904 formed on piezoelectric substrate 902, reaction section 905 formed on a propagation path between transmitting electrode 903 and receiving electrode 904 on piezoelectric substrate 902, and a detector detecting a characteristic of a main acoustic wave excited by transmitting electrode 903.
When a substance (such as breath or inspection liquid, etc.) which may possibly contain an object to be detected is injected on reaction section 905, the detector detects a change in a frequency of an acoustic wave due to the attaching of the object, thereby allowing the acoustic wave sensor to detect the presence of the object or a concentration thereof.
A conventional acoustic wave sensor similar to acoustic wave sensor 901 is described in Patent Literature 2.
As acoustic wave sensor 901 has a smaller size, the area of reaction section 905 decreases, so that the amount of the object adsorbed on reaction section 905 reduces, accordingly deceasing detecting precision of the acoustic wave sensor 901.
The acoustic wave sensor is used to examine or analyze liquid, which is a sample, or a sample (substance to be examined) contained in liquid. The acoustic wave sensor includes a transmitting/receiving electrode portion for transmitting or receiving an acoustic wave on a piezoelectric substrate. The acoustic wave sensor utilizes a characteristic that the propagation characteristic of the acoustic wave propagating on the piezoelectric substrate changes according to the state of the surface of the piezoelectric substrate on which the acoustic wave is propagated. The acoustic wave sensors described above is used in various fields, such as medical, environmental, and food fields.
For example, a sensor including with a detection area for detecting a sample on a propagation path of an acoustic wave between transmitting/receiving electrode portions is used as a biosensor. In a general acoustic wave sensor, a metal layer is formed on the detection area. In the biosensor, an antibody layer is further formed on the metal layer. When antigen solution is injected into the antibody layer, the biosensor collects antigens by an antigen-antibody reaction. Therefore, the propagation characteristic of the acoustic wave propagating on the piezoelectric substrate is changed before and after the injection of the antigen solution. Accordingly, the amount of the collected antigens or the concentration of the antigen can be detected by detecting the change in the propagation characteristic of the acoustic wave before and after the injection of the solution.
In the acoustic wave sensor described above, the transmitting/receiving electrode portion is generally covered by a cover for preventing the transmitting/receiving electrode portion from being affected by ambient atmosphere, such as moisture, in order to prevent the deterioration in detecting sensitivity and reproducibility. The cover may be implemented by a case sealing the transmitting electrode portion and the receiving electrode portion to form a hollow space serving as a vibration area on both the electrode portions, or by a dielectric layer formed on the transmitting electrode portion and the receiving electrode portion.
FIG. 22A is a schematic top view of still another acoustic wave sensor 950. FIG. 22B is a schematic cross-sectional view of acoustic wave sensor 950 on line 22B-22B shown in FIG. 22A. In acoustic wave sensor 950, transmitting electrode portion 814 is covered and protected by transmitting electrode cover 812 while receiving electrode portion 815 is covered and protected by receiving electrode cover 813.
This type of the acoustic wave sensor is described in Patent Literature 3.